So is lithium crystals, anti-matter destroying the universe, being able to create two people of the same mass out of one person, being able to merge two being into one that has equal mass of the two and so on...

Just because it's canon doesn't mean it makes sense.

"The Enemy Within" presents a very similar situation, but I never see anyone arguing that Evil Kirk has a right to live. Except Evil Kirk's objections: "I wanna live!"

Yeah. I brought that up earlier in the thread but it doesn't fit anyone's pro-Tuvix arguments so it got ignored.